快猫短视频

WE NEED friends. They have a positive impact on our health, wealth and mental well-being. Social isolation, on the other hand, creates feelings akin to physical pain and leaves us stressed and susceptible to illness. In fact, our bodies react to a lack of friends as if a crucial biological need is going unfulfilled. This is not surprising. For us humans, friends are not an optional extra 鈥� we have evolved to rely on them.

But friendship comes at a cost; time spent socialising could be used in other activities key to survival such as preparing food, having sex and sleeping. Besides, just because something is good for us, doesn鈥檛 mean we will necessarily do it. That鈥檚 why evolution has equipped us with the desire to make friends and spend time with them. Like sex, eating or anything a species needs to survive, . In other words, being friendly is linked with the release of various neurotransmitters in the brain and biochemicals in the body that make us feel good.

Understanding what motivates friendship begins in a seemingly unlikely place 鈥� with lactation. As a baby suckles, a neuropeptide called oxytocin is released from the mother鈥檚 pituitary gland. This causes muscles in the breast to contract, allowing milk to flow, but it also reduces anxiety, blood pressure and heart rate. For mothers and babies, the relaxed feeling produced by oxytocin encourages suckling and helps create a strong and loving bond. This occurs in all mammals, but in humans and the few other species that make friends the system has been co-opted and expanded. Rather than reinvent the wheel, evolution has economised and oxytocin has become associated with relationships beyond the mother-child bond. You release it in response to many types of positive physical contact with another person, including hugs, light touches and massage. The resulting pleasant feeling is your reward for the interaction and encourages you to see that person again. A budding friendship is born.

Of course, most interactions between friends do not involve physical contact but oxytocin works in other ways too. It promotes prosocial decisions, increases feelings of trust and encourages generosity. And, while important, it is not the only chemical driver of friendship. Another key player is a group of opioid chemicals called endorphins. Also produced by the pituitary gland, they are released in response to mild pain, such as exercise, and act as neurotransmitters in the brain to create a feeling of well-being. All vertebrates produce endorphins, so they must have evolved early on, but like oxytocin, they have come to play a role in motivating friendship. Endorphins also make physical contact feel good, but they underpin another aspect of friendship too.

Robin Dunbar and his colleagues at the University of Oxford asked people to row a boat, either alone or in pairs, and measured their endorphin levels before and after. What they found was striking. Despite exerting the same physical effort, people who rowed as a . One of the major components of friendship is behavioural synchrony 鈥� friends must be in the same place at the same time to establish and maintain a relationship. Endorphins seem to promote friendships by making synchrony feel good.

The flip side of this is how bad it feels to be socially isolated. Lonely people have elevated levels of the stress hormone cortisol. Chronic stress damages your health, which probably explains why social isolation increases the risk of cardiovascular diseases and susceptibility to infection. But stress can be useful. The stress response is produced by activation of a system known as the hypothalamic-pituitary-adrenal axis. This activation acts as a warning that homeostasis 鈥� the body鈥檚 maintenance of stable internal conditions 鈥� has been disrupted. So stress prompts us to behave in ways that restore homeostasis, including resting when tired and seeking shade when hot. Perhaps it also motivates us to seek out social contact when we are lonely. The fact that suggests that friends either help us to restore homeostasis or prevent its disruption in the first place.

鈥淪tress may motivate us to seek social contact when we are lonely鈥�

To select, acquire and maintain friends we need to gather social information. Again, this is something we enjoy. Even before babies can speak, they prefer looking at faces than other visual stimuli. We find social information intrinsically rewarding because it triggers reward-related areas of the brain. When Dar Meshi of the Free University in Berlin, Germany, showed people in an MRI scanner pictures from their Facebook accounts, , a brain region associated with drug addiction. Interestingly, people with the greatest response were the most frequent social media users.

Although the neural and biochemical processes that underlie friendship are the same in everyone, some people are friendlier than others. These people may simply be better at making friends, but Meshi鈥檚 results hint that they are also more motivated to do so because it gives them a bigger kick. Friendlier people are more sociable, in part, because their genes make them that way. James Fowler at the University of California, San Diego, and Nicholas Christakis at Harvard University compared the social networks of identical twins, who share all their genes, and paternal twins, who share 50 per cent on average. They found that .

Even social butterflies aren鈥檛 friends with everyone. Of the many people we encounter, how do we pick a select few? The answer, at first, seems quite simple 鈥� we are friends with people who are similar to us, whether they are the same age, gender or profession. But it turns out that this tendency for 鈥渓ike鈥� to associate with 鈥渓ike鈥�, termed homophily, also has a basis in our genes. Fowler and Christakis found that . One of the mysteries of friendship has been why we would cooperate so readily with complete strangers. In evolutionary terms, you should cooperate with kin rather than kindred spirits because your genetic similarity to relatives allows you to reap indirect benefits. In other words, you succeed by proxy if they pass on more of the genes they share with you to future generations. But if friends are more genetically similar than we would expect by chance, perhaps we should think of them not so much as strangers than as 鈥渇acultative relatives鈥�.

鈥淔riends are as genetically similar to each other as fourth cousins are鈥�

So, your genome may help determine not only how friendly you are, but also who you choose for your friends. No one knows how we recognise people who are genetically similar. It could be similarities in facial features, voice, gestures or smell. Our tendency to befriend people who share our traits may even hold an answer. Your personality is shaped in part by your genes so, if you choose friends with a similar personality, they will probably have genes in common with you. Whatever attracts us to certain people, one thing is certain, befriending them will be rewarding. Because if there鈥檚 one thing we all know about friendship, it鈥檚 that it feels good.

Read more: 鈥�Friendship: The chemistry of our social glue鈥�

This article appeared in print under the headline 鈥淔riends with benefits鈥�